Extended Source Subtraction

This is a much trickier problem. The asthelp section "xrtsub advice extended_sources" gives up to date advice on the best subtraction method. The subtraction of an image provides a good illustration of the current best technique. Let's say a background subtracted image of the whole field between corrected pulse height channels 11 and 250 is required. First, xrtsort should be used to make a spectral_image over this channel range with about 6 spectral bins. A few spectral bins need to be used to adequately sample the vignetting function which is energy dependent. (It is unnecessary to use more than about 10 spectral bins because of the moderate energy resolution of the instrument. Indeed using a large number of energy bins can cause problems due to irregularities in the vignetting function).

The background area needs to be large but constrained within a narrow range of off-axis angles (so that a single vignetting correction may be applied to the data). The best shape to satisfy these criteria is an annulus at a reasonably large off-axis angle (e.g. r=0.6^o - 0.7^o). To produce such an annulus with sources and window support structure removed, use the following procedure:

• Mask support structures using xspokes and ardqual.
• Select your background region using iregion annulus.
• Export the remaining areas using iregion invert and iregion export.
• Mask the un-selected regions using ardqual.

xrtsub may then be run on the source and background spectral images. The output spectral image can be compressed into an image using project. Alternatively it may be projected into a spectrum for spectral work. NB: DO NOT compress the background spectral image into a spectrum before background subtraction as this will result in the spatial information being lost and the program will be unable to normalise for the box areas.

The above procedure is fairly robust against spatial variations in the background. One problem is that beyond the central ring, it becomes harder to resolve point sources from the background. This means that faint sources may be included in the background region when they would have been ignored in the middle. This can cause a slight over-subtraction and be a problem for very accurate work. No data should be used beyond a radius of 50 arcminutes due to inaccuracies in the vignetting function at these large radii.